A. Field of the Invention
This invention relates to the design of gear teeth. Specifically, it relates to gearing having characteristics that reduce the self-excited component of dynamic load. As a consequence, this gearing is quieter and exhibits greater power density than conventional gearing. The invention discloses a gear form that achieves these desirable performance characteristics in a way that is less costly and, in many commonly encountered circumstances, more robust than previously disclosed gear forms.
B. Related Art
It is well known in the art that, as a pair of gears rotates, the effective stiffness of the gear mesh varies as a function of mesh cycle position. This periodic variation in mesh stiffness gives rise to what has been referred to in the art as the "dynamic increment" of load. In addition to serving as the primary excitation for gear noise, this "dynamic increment" serves to reduce effective torque capacity. Any means that can be employed to reduce this self-excited component of dynamic load will have the effect of reducing gear noise and increasing power density.
U.S. Pat. Nos. 5,083,474, 5,341,699 and 5,485,761, disclose an elastic means for reducing mesh stiffness variation that is effective at all operating loads. This desirable mesh stiffness characteristic is achieved by introducing different forms and amounts of lead crowning at different positions along the length of the gear tooth. The introduction of this "differential crowning" serves to decrease the elastic stiffness of gear teeth in the portion of the mesh cycle having the greater number of tooth pairs in contact. While effective in reducing mesh stiffness variation, this approach, which frequently concentrates loads over a small area, can lead to excessive contact pressures and stresses, potentially compromising performance in high-speed or high-load applications. Furthermore, the manufacture of gears having different forms and amounts of crowning at different roll angle positions often requires special cutting tools and/or manufacturing processes.
U.S. Pat. No. 5,315,790 discloses a method of using finite element analysis to calculate a tooth topology that compensates for load-induced tooth deflections. Similar in effect to conventional tip relief, and unlike the means disclosed in the present application, this type of modification uses initial separation to reduce mesh stiffness variation; as a consequence, it is effective only in the vicinity of a particular design load. In addition, this particular type of modification can be difficult and expensive to produce. U.S. Pat. No. 5,802,921 discloses the use of "liminal cross-modification" for reducing dynamic loading and tooth impact in helical and spiral bevel gearing only. Although this approach does not lead to an increase in contact pressure, the means used to achieve this end are markedly different from and typically more expensive than those prescribed by the present invention.